1. Technical Field
The invention is useful in Code Division Multiple Access mobile cellular or satellite communications systems employing orthogonal coding on at least one of the forward link (downlink) or return link (uplink), and can be used in particular to avoid the pilot code of one cell interfering with reception in a neighboring cell.
2. Related Art and Other Considerations
Research shows that the capacity of a cellular communications system is maximized when no own-cell interference exists; that is, signals in the same cell do not interfere with one another, and the only interference is from other cells. There are two basic methods for avoiding own-cell interference:
A first method for avoiding own-cell interference transmits signals in the same cell that are not orthogonal to each other, i.e. they have a non-zero correlation one with another, but uses some type of interference cancellating receiver such as a joint demodulator to avoid or reduce the interference effects.
A second method for avoiding own-cell interference attempts to keep signals in the same cell orthogonal to each other either by giving them different frequencies (FDMA), different transmission timeslots (TDMA) or different orthogonal spreading codes (CDMA).
The orthogonality of TDMA and FDMA systems is unaffected by propagation delay differences or multipath, while the orthogonality of CDMA is progressively destroyed as the spread bandwidth increases. TDMA is better suited for a mobile communications system than FDMA or CDMA, as motion-induced channel changes are slower in relation to the underlying information symbol modulation rate.
Therefore unless interference cancellation is used (e.g., the first method), TDMA will exhibit higher capacity than CDMA, other factors that affect the comparison, such as power control, error correction coding etc being alike.
In U.S. Pat. Nos. 6,215,762 and 6,359,874 (incorporated herein by reference), a block-spreading system is disclosed that is better at maintaining orthogonality under multipath conditions, allowing the CDMA systems of the second above-described method to approach the capacity of TDMA systems. U.S. Pat. No. 5,151,919 (incorporated herein by reference) discloses a first above-described method system (interference cancellation) which may achieve even higher capacity at the expense of greater receiver complexity.
The term Code Division Multiple Access (CDMA) refers to a method by which multiple transmitting stations (e.g. mobile phones) can Communicate their independent data streams to a common receiver (e.g. a cellular base station receiver) The communication at multiple data streams from a common transmitter (i.e. a cellular base station) to a plurality of receivers (i.e. different mobile phones) by Code Division is perhaps more properly referred to as Code Division Multiplex. However, for sake of simplicity the two terms Code Division Multiple Access and Code Division Multiplex will herein be regarded as equivalent. Thus “CDMA” will be used as a term of description for both the uplink and the downlink, even though the downlink may be more accurately described as Code Division Multiplex (CDM).
The use of orthogonal Walsh codes in communications systems is well known. The Code Division Multiple Access system known as IS95 uses Walsh codes of length 64 to scramble signals transmitted by a cellular base station to different mobile stations, the orthogonality of the codes reducing interference between signals intended for different mobiles. The IS95 return link uses all 64, length-64 Walsh codes in every mobile station to encode 6-bit data blocks, the different mobile transmissions being rendered unlike by the use of different scrambling sequences. This return link or uplink use of the Walsh codes does not attempt to make different signals orthogonal, but rather is a farm of error correction coding called (64,6) orthogonal block encoding. See, for example, Garg, Vijay K. et al., Applications of CDMA in Wireless/Personal Communications, Prentice Hall (1997), which is incorporated herein by reference.
The wideband CDMA cellular system known as UMPTS also uses orthogonal Walsh codes to discriminate between different base-to-mobile links. In this case, the orthogonal codes are not of a fixed length, but depend on the datarate of a particular link. Nevertheless, signals of different code length and datarate remain nominally orthogonal to each other.
In both IS95 and UMPTS, different cells apply different scrambling sequences to transmissions originating in the cell, so that no transmission in one cell is spread identically to a transmission in an adjacent cell. The transmissions in one cell thus appear as noise to the receivers in another cell.
The total interfering power from one cell to an adjacent cell is thus partly due to high power transmissions between the base station and cell-edge stations, and partly due to lower power transmissions between the base station and mobile station This effect known as “interference averaging”, is a desirable effect that avoids the need to design for worst cases.
When randomly different scrambling codes are used to make transmissions in neighboring cells different, there is a statistical probability that the correlation between a code in one cell and a code in the other cell will momentarily be high, departing from ideal interference-averaging statistics. To improve the statistics, one or more of U.S. Pat. Nos. 5,353,352; 5,550,809; 5,742,678; and 5,771,288 (all incorporated herein by reference) disclose a method of devising systematically different scrambling codes for use in neighboring cells that provide a guaranteed low correlation between any code used for one transmission and any code used for another transmission.
In CDMA systems, it is common to reserve a particular code in each cell for transmitting a reference or “pilot” signal from the base station that can be used as a phase and channel reference for decoding traffic signals using other codes. As much as 25% of the transmitted power from a base station can be allocated to such a reference signal. When the traffic loading in a cell is low, the reference signal can thus often be the majority of the interference exported to neighboring cells, a phenomenon known as “pilot pollution”. It would be desirable if the pilot signal of one cell could either be the same in all cells, as is possible for satellite systems, or be at least nominally orthogonal to the traffic signals in another cell to avoid such interference. By nominally orthogonal is meant that the correlation is zero with zero time difference between the signals of one cell and the signals in another cell. When a pilot code common to all cells is employed, the pilot is desirably orthogonal to all traffic signals in all cells. This is more readily achieved in a satellite system, where multipath propagation is limited, and the distance and channel through which signals propagate from the satellite to any given station is identical for all signals. It is also more feasible to achieve time synchonisation between the sectors of a sectorized cellular system, wherein the service area of a base station is divided into angular sectors served by directional antennas on the same tower.
Thus, particularly in satellite applications of CDMA, there is an interest in having pilot codes in one cell be orthogonal to traffic signals in another cell. This requires other methods than using different scrambling codes to render the traffic codes in one cell different than the traffic codes used in another cell.
For terrestrial applications, other methods to preserve orthogonality in the face of multipath propagation or delay differences are disclosed in the aforementioned U.S. Pat. Nos. 6,215,762 and 6,359,874. Such methods may be improved by using the current invention to construct codesets that have codes in common with the codes used in adjacent cells.